LNK302/304-306
The LinkSwitch-TN oscillator incorporates circuitry that
introduces a small amount of frequency jitter, typically 4 kHz
peak-to-peak, to minimize EMI emission. The modulation rate
of the frequency jitter is set to 1 kHz to optimize EMI reduction
for both average and quasi-peak emissions. The frequency
jitter should be measured with the oscilloscope triggered at
the falling edge of the DRAIN waveform. The waveform in
Figure 4 illustrates the frequency jitter of the LinkSwitch-TN.
Pin Functional Description
DRAIN (D) Pin:
Power MOSFET drain connection. Provides internal operating
current for both start-up and steady-state operation.
BYPASS (BP) Pin:
Connection point for a 0.1 μF external bypass capacitor for the
internally generated 5.8 V supply.
Feedback Input Circuit
FEEDBACK (FB) Pin:
The feedback input circuit at the FB pin consists of a low
impedancesourcefolloweroutputsetat1.65V.Whenthecurrent
deliveredintothispinexceeds49μA,alowlogiclevel(disable)
is generated at the output of the feedback circuit. This output
is sampled at the beginning of each cycle on the rising edge of
the clock signal. If high, the power MOSFET is turned on for
thatcycle(enabled), otherwisethepowerMOSFETremainsoff
(disabled). Since the sampling is done only at the beginning of
each cycle, subsequent changes in the FB pin voltage or current
during the remainder of the cycle are ignored.
During normal operation, switching of the power MOSFET is
controlled by this pin. MOSFET switching is terminated when
a current greater than 49 μA is delivered into this pin.
SOURCE (S) Pin:
This pin is the power MOSFET source connection. It is also the
ground reference for the BYPASS and FEEDBACK pins.
P Package (DIP-8B)
G Package (SMD-8B)
5.8 V Regulator and 6.3 V Shunt Voltage Clamp
D Package (SO-8C)
The 5.8 V regulator charges the bypass capacitor connected to
the BYPASS pin to 5.8 V by drawing a current from the voltage
on the DRAIN, whenever the MOSFET is off. The BYPASS
pin is the internal supply voltage node for the LinkSwitch-TN.
When the MOSFET is on, the LinkSwitch-TN runs off of the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows the LinkSwitch-TN
tooperatecontinuouslyfromthecurrentdrawnfromtheDRAIN
pin. A bypass capacitor value of 0.1 μF is sufficient for both
high frequency decoupling and energy storage.
S
S
S
S
1
2
8
7
1
2
8
7
BP
FB
S
S
S
S
6
5
BP
FB
3
4
4
D
5
D
3a
3b
PI-3491-120706
In addition, there is a 6.3 V shunt regulator clamping the
BYPASS pin at 6.3 V when current is provided to the BYPASS
pin through an external resistor. This facilitates powering of
LinkSwitch-TN externally through a bias winding to decrease
the no-load consumption to about 50 mW.
Figure 3. Pin Configuration.
LinkSwitch-TN Functional
Description
BYPASS Pin Under-Voltage
LinkSwitch-TNcombinesahighvoltagepowerMOSFETswitch
withapowersupplycontrollerinonedevice.Unlikeconventional
PWM(pulsewidthmodulator)controllers,LinkSwitch-TNuses
a simple ON/OFF control to regulate the output voltage. The
LinkSwitch-TN controller consists of an oscillator, feedback
(sense and logic) circuit, 5.8 V regulator, BYPASS pin under-
voltagecircuit,over-temperatureprotection,frequencyjittering,
current limit circuit, leading edge blanking and a 700 V power
MOSFET.TheLinkSwitch-TNincorporatesadditionalcircuitry
for auto-restart.
The BYPASS pin under-voltage circuitry disables the power
MOSFET when the BYPASS pin voltage drops below 4.85 V.
Once the BYPASS pin voltage drops below 4.85 V, it must rise
back to 5.8 V to enable (turn-on) the power MOSFET.
Over-Temperature Protection
The thermal shutdown circuitry senses the die temperature.
The threshold is set at 142 °C typical with a 75 °C hysteresis.
Whenthedietemperaturerisesabovethisthreshold(142°C)the
power MOSFET is disabled and remains disabled until the die
temperature falls by 75 °C, at which point it is re-enabled.
Oscillator
The typical oscillator frequency is internally set to an average
of 66 kHz. Two signals are generated from the oscillator: the
maximum duty cycle signal (DCMAX) and the clock signal that
indicates the beginning of each cycle.
Current Limit
ThecurrentlimitcircuitsensesthecurrentinthepowerMOSFET.
When this current exceeds the internal threshold (ILIMIT), the
23-3
Rev. I 11/08
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